Porewater advection, transport

Jahnke, R.A., Nelson, J.R., Marinelli, R.L., and Eckman, J.E. (2000) Benthic flux of biogenic elements on the southeastern U.S. continental shelf influence of porewater advective transport and benthic microalgae. Cont. Shelf Res. 20, 109-127. 

The typical application of these models is similar to those presented above except that porewater flow is characterized by a Darcian velocity v (m/s) so that advective transport is superimposed on the diffusive transport. The sohd is infinite in extent and the processes are transient with transport occurring in the y-direction. 

The subject of advective porewater flux and associated chemical transport is covered in Chapter 11 in the context of aquatic bed-sediment systems, which include surface soil-derived particle layers on the bottom of streams, rivers, lakes, estuaries, and the near-shore marine environment. This soil system is a saturated porous medium and therefore the fundamentals of the transport processes and related parameters within this system are identical to that of surface soils. A brief review of advective transport in the subsurface follows. 

This chapter is concerned with molecular diffusion-related chemical transport parameter estimation procedures at the water-sediment interface. Transport coefficients for particle resuspension and settling are in Chapter 10. Chapter 11 is concerned with the bed porewater advective process and Chapter 13 covers macrofauna driven... 

It is desirable to calculate new bulk phase Z values for the four primary media which include the contribution of dispersed phases within each medium as described by Mackay and Paterson (1991) and as listed earlier. The air is now treated as an air-aerosol mixture, water as water plus suspended particles and fish, soil as solids, air and water, and sediment as solids and porewater. The Z values thus differ from the Level I and Level II pure phase values. The necessity of introducing this complication arises from the fact that much of the intermedia transport of the chemicals occurs in association with the movement of chemical in these dispersed phases. To accommodate this change the same volumes of the soil solids and sediment solids are retained, but the total phase volumes are increased. These Level III volumes are also given in Table 1.5.2. The reaction and advection D values employ the generally smaller bulk phase Z values but the same residence times thus the G values are increased and the D values are generally larger. 

Reaction rates of nonconservative chemicals in marine sediments can be estimated from porewater concentration profiles using a mathematical model similar to the onedimensional advection-diffusion model for the water column presented in Section 4.3.4. As with the water column, horizontal concentration gradients are assumed to be negligible as compared to the vertical gradients. In contrast to the water column, solute transport in the pore waters is controlled by molecular diffusion and advection, with the effects of turbulent mixing being negligible. 

As a result of particle settlement to the sediment-water interface, there is a mass accumulation of sediments which results in compaction of sediments and the physical upward transport or advection of solutes in porewaters to the overlying water. Similarly, solutes in porewaters can also move by diffusion as a result of concentration gradients. Thus, porewaters can be transported by advection from burials, molecular diffusion, and biological pumping or irrigation (Aller, 2001 Jprgensen and Boudreau, 2001). Diffusion in aqueous environments occurs according to Fick s laws of diffusion (Berner, 1980). Fick s first law, used for steady-state conditions, is as follows ... 

Asa result of particle settlement to the sediment-water interface there is a mass accumulation of sediments which results in compaction of sediments and the physical upward transport or advection of solutes in porewaters to the overlying water. Similarly, solutes in porewaters can also move by diffusion as a result of concentration gradients. 

In particular, horizontal advection and horizontal diffusion in the Chesapeake Bay are comparable while vertical difiiision is a fast process that acts over short distances, and a model must account for all three. In this environment, atrazine that is discharged to the surface waters could be horizontally distributed over a distance of 1 km over a period of one week, since the time scale of horizontal advection-difiusion processes is 10 -10 s (approximately 3 hours). As atrazine is distributed horizontally, it also mixes vertically down the water coluitm. With the estimates of verticd diffiisivity for the Bay that are available in the literature, for a depth of 10-20 m the time scale for vertical diffusion processes is on the order of 15 minutes, and can be as short as 3 minutes. The sidfidic vraters are in the sediment porewaters and atrazine needs to be transported to the water-sediment inter ce in order to encounter and react with reduced sulfiir species. The characteristic horizontal and vertical scales that describe the flow in the Bay indicate that it is possible for atrazine to reach the depth of the water-sediment interface before it is horizontally transported out of the system. The subsequent exchange at the water-sediment interface depends on many factors, including half-life of atrazine, the hydraulic residence time of the bottom layer, turbulent processes, and other characteristics of the water column above the sediment layer. Simple box models cannot capture the dynamics necessary to describe these exchanges that ultimately govern the te of atrazine in the Bay. 

A total of 41 individual transport processes are listed in Table 4.1 as being the most significant ones of concern in this handbook. Seven in the hst are soil-side transport processes and the main focus of Part 2 of this chapter. Several of these processes including diffusion, advection, and fluid-to-solid mass transfer in porous media are also relevant to many other environmental compartments and are covered in more detail in other chapters of this book. Specifically the chapters are mass transport fundamentals from an environmental perspective (Chapter 2) molecular diffusion estimation methods (Chapter 5) advective porewater flux and chemical transport in bed-sediment (Chapter 11) diffusive chemical transport across water and sediment boundary layers (Chapter 12) bioturbation and other sorbed-phase transport processes in surface soils and sediments (Chapter 13), and dispersion and mass transfer in groundwater of the near-surface geologic formations (Chapter 15). 

Advective Porewater Flux and Chemical Transport in Bed-Sediment... 

This chapter focuses on a parameter that regulates the magnitude of the mass-transport coefficient for determining chemical flux across sediment deposits in aquatic systems. Owing to the interactions between the groundwater and the surface water, advective porewater flux moves soluble and colloid-bound chemicals across this environmental interface. Deposited sediments form a dynamic layer of porous solid material on... 

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